Review of the thermoelectric properties of layered oxides and chalcogenides

The current state of investigation on thermoelectric properties of layered chalcogenides and oxides is considered. The relationship between the quasi-two-dimensionality of electronic transport properties and thermoelectric properties is confirmed. A decrease in the dimension of electron transport from three-dimensional to quasi-two-dimensional in materials with a layered structure increases the thermopower with a slight change in electrical conductivity. The bismuth tellurides, bismuth selenides and its alloys are currently one of the outstanding state of the art bulk thermoelectric materials with layered structure. Layered transition metal dichalcogenides MX$_2$ (M is a transition metal, X is a chalcogen) are efficient thermoelectric materials at higher temperatures (500-800 K). In these materials, an increase in thermoelectric properties associated with the two-dimensionalization of electron transport is also observed. Layered monochalcogenides MX (M = Sn, Pb, Ge; X = S, Se, Te) are also a promising class of thermoelectric materials. In SnSe single crystals, $ZT\sim$ 2.6 is obtained at 923 K due to the very low thermal conductivity along the $b$ axis (0.23 W/(m$\cdot$K) at 973 K). Layered chalcogenides CuCrX$_2$ (X - S, Se, Te) containing magnetic Cr atoms are effective thermoelectrics at higher temperatures (up to 800 K) due to the presence of phonon glass – electron crystal state led to a significant decrease in thermal conductivity at high temperatures. Magnetic atoms in CuCrX$_2$ compounds lead to the presence of magnetic phase transitions affecting their thermoelectric properties. Interest in oxide-based thermoelectric materials has significantly increased due to their stability in air and higher temperatures for maximum efficiency. The most promising thermoelectric oxide materials are Ca$_3$Co$_4$O$_9$, Na$_x$CoO$_2$, Bi$_2$Ca$_2$Co$_2$O$_x$, and CaCo$_2$O$_4$ have a layered structure and contain magnetic Co atoms leading to magnetic ordering and influence on thermoelectric properties. The presence of phase transitions affects the thermoelectric parameters of thermoelectrics and the thermoelectric figure of merit $ZT$.

Synthesis and study of a novel 9P-type mixed layered tetradymite-like GeBi4Te4 compound in the Ge-Te-Bi system

Taking as a base structural properties of the tetradymite-like layered chalcogenide compounds, a novel ternary GeBi4Te4 phase was predicted and synthesized. The polycrystal of this compound was synthesized and its single-crystal was grown by the vertical Brigman-Stokbarger method. The phase composition of obtained samples was analyzed by differential thermal analysis (DTA), X-ray powder diffraction (XRD), and scanning electron microscope (SEM). It was determined that the novel phase melts peritectially at 538 °C and has the following lattice parameter values in a hexagonal configuration: a=4,4071(6)Å, c=17,384(2)Å (sp. gr P3m1). This crystal structure is considered a derivative of tetradymite-like layered chalcogenides and consists of alternating Bi bilayers and GeBi2Te4 septuple packets.

Influence of the stacking sequence on layered-chalcogenides properties: First principle investigation of Pb2Bi2Te5

The Pb2Bi2Te5 compound has been reported in literature with two stacking sequences -Te-Pb-Te-Bi-Te-Bi-Te-Pb-Te- and -Te-Bi-Te-Pb-Te-Pb-Te-Bi-Te- labelled in this work as A and B, respectively. The electronic and the thermoelectric properties...